This invention is directed toward an extremely sensitive, high bandwidth force transducer operating on the Michelson interferometric principle and is capable of measuring minimal deflection of a load beam. More particularly, this invention has application in the study of muscle mechanics involving a single muscle fiber having high resonant frequencies and exerting minimal load forces.
In a number of experimental situations involving whole muscle and single skeleton muscle fibers, it is necessary to achieve extremely high force transduction fidelity over a large bandwidth of frequencies where measuring extremely small deflections is critical. Because force transducers typically measure displacement of a component having a known response to an applied force, the sensitivity and responsiveness of a force transducer depends upon the magnitude of deflection of the loaded element. When the applied force is very low or the force is applied to effectuate extremely small deflection in the loaded element and the bandwidth is relatively large, the inherent difficulties in measuring force are compounded.
There are a number of force transducers known in the art, but these transducers are not particularly well suited to measuring small forces or a minimal amount of displacement of the loaded member. For example, resistance strain gauges typically measure the change in resistance of a circuit resulting from deflection applied to an electrical path through the loaded member. Resistive strain gauges require large deflections and have relatively low signal-to-noise ratios. Another type of force transducer is the variable capacitance transducer which manifests a change in separation between capacitive plates through a change in the capacitance of a circuit. Capacitive transducers tend to be extremely sensitive to environmental conditions such as humidity and temperature, require sophisticated electronic equipment to detect the change in capacitance, and tend to capacitively couple to their surroundings. Yet another type of force transducer is the variable inductance based transducers which exhibits a change in applied force through a change of inductance in an electrical circuit. Inductance based transducers, however, tend to have an extremely limited frequency response. Finally, piezo-electric force transducers are less sensitive, do not respond well to low frequency inputs, and have a relatively high mechanical hysteresis. In addition to the above mentioned force transducers, several others use optical deflection or occlusion of a light path as the transducing element. Such deflection or occlusion results in a significant compromise between resonant frequency and displacement.
This apparatus is directed to a force transducer that addresses the limitations described above. In particular, the apparatus provides a force transducer having a much higher sensitivity, resonant frequency, response time, output voltage, and signal-to-noise ratio in combination than any of the above mentioned transducers. This apparatus has the added benefit that it is not capacitively or inductively coupled to the environment.
According to one embodiment of this invention, laser light is directed at a beam splitter of a Michelson interferometer. The beam splitter transmits light in the direction of a stationary reflective surface and also reflects light in the direction of a movable reflective surface. The movable reflective surface is applied to a load member to which a force is applied. When a force displaces the movable reflective surface, the reflected light from the stationary and the movable reflective surface interact to define an interference fringe pattern. The shift in one fringe of the interference fringe pattern over a substantially linear range of the fringe is measured to determine the displacement of the load member. The magnitude of the fringe displacement is in accordance with the magnitude of the force applied to the load member. The reflective surfaces of the interferometer may be applied directly to the members of the interferometer, rather than using a mirror, in order to further reduce the effects of temperature.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.